1. Field of the Invention
The present invention relates to a process and apparatus for applying particles to a substrate, to a process for forming a layer on a substrate, to substrates having particles, and to substrates having thin layers. In another aspect, the present invention relates to a process and apparatus for applying charged particles to a substrate, to a process for forming the charged particles into a layer, to substrates having charged particles, and to substrates having a thin layer. In even another aspect, the present invention relates to a process and apparatus for applying charged particles to a substrate at a density greater than 1011 paricles per cm2, to a process of forming the particles into a layer, to a substrate coated with greater than 1011 charged particles per cm2, to a substrate having a layer having a nucleation density greater than 1011 paricles per cm2. In still another aspect, the present invention relates to a process and apparatus for applying charged diamond particles to a silicon substrate, to a process of forming the diamond particles into a layer on the silicon substrate, to silcon substrates having charged diamond particles, and to a silcon substrate having a thin diamond layer. In yet another aspect, the present invention relates to a process and apparatus for applying charged diamond particles to a silicon substrate at greater than 1011 particles per cm2, to a process of forming the particles into a diamond layer on the silicon substrate, to silcon substrates having charged diamond particles at a density of greater than 1011 diamond particles per cm2, and to a silcon substrate having a thin diamond layer with nucleation density greater than 1011 particles per cm2.
2. Description of the Related Art
A. Diamond Film Nucleation
Due to a unique combination of its mechanical, physical, chemical, and electrical properties, diamond is an excellent material for a variety of applications. A wide range of applications which will utilize the exceptional mechanical, thermal, optical, and electronic properties of diamond are anticipated for thin diamond films. Examples include coating of mechanical tools and optical windows, protective layers on intergrated circuits, heat spreaders for high power electronic devices, microsensors, cold emitters, and high temperature semiconductor devices. The diamond films for these applications need to be prepared with specific desired microstructure, thickness, and reproducibility within the practical limits of time and expense.
The nucleation of diamond growth on a substrate depends on the substrate material on which the diamond is to be grown. The initiation of diamond growth is also affected by the surface conditions of the substrate. Unfortunately, formation of diamond films on non-diamond substrates has proved extremely difficult. It has long been understood that the low-nucleation density behavior of diamond is one of the key factors responsible for poor microstructure and morphology of films on non-diamond substrates. Lower nucleation densities require longer deposition times for continuous films and, consequently, result in large surface roughness due to large grain sizes, with problems being more pronounced for highly oriented films and low temperature growth.
From a technological point of view, silicon, as the basic material for present electronics, is strongly favored as a substrate material for thin film diamond devices. Consequently, diamond growth on silicon has received substantial attention in the prior art. However, owing to the high surface energy of diamond and the relatively low sticking probability of the precurser for diamond nucleation, diamond nuclei grow very poorly on a mirror-polished silicon substrate.
In general, the diamond films produced consist of polycrystalline, highly defective, randomly-oriented diamond crystals containing varying amounts of non-diamond carbon and hydrogen. Furthermore, a large number of microvoids (or microcavities) is present in CVD diamond films due to the large grain size of the crystallites (columnar growth) in combination with a low density of grains. The control of the microstructure of diamond films depends, to a great extent, on the nucleation density, and consequently, it requires control of the nucleation process. The deposition of diamond is enhanced by increasing the number of nucleation sites on the substrate, increasing the carbon content in the hydrogen plasma, or reinforcing the bonds between nuclei and the substrate. Failure to provide a high nucleation density results in polycrystalline diamond films with very rough surfaces and/or a porous core.
Diamond-nucleation density on a polished silicon surface is generally on the order of 104 per cm2. The ability to create an extremely high nucleation density for the growth of diamond films is a key problem for the realization of electrical and optical quality synthetic diamond.
Prior art methods exist to enhance diamond nucleation several orders of magnitude into the range of 107 to 1011 per 2 cm. These prior art methods for enhancing diamond nucleation density on such a mirror-polished silicon substrate generally include (1) scratching or abrading the surface of the substrate; (2) creating submicrometer-craters by focused ion beams; (3) electrophoretic deposition of diamond monolayers (4) sputtering; (5) preferentially etching with a chemical solution to create micrometer-scale V-grooves; (6) coating with a low vapor pressure and high thermal stability hydrocarbon oil; (7) coating with a thin layer of evaporated carbon; (8) applying a substrate bias voltage; (9) spin-coating of photoresist containing seed particles, and (10) Si+ion beam implantation.
B. Machine Tools
The coating of machine tools (substrates) with diamond for enhanced cutting and operating life has been pursued for several years. The conventional approach to coating of substrates is by microwave chemical vapor deposition or hot filament chemical vapor deposition. Although these methods work to grow nearly pure diamond, they are expensive and/or slow. The initiation of chemical vapor diamond growth (i.e., nucleation) is affected by the surface conditions of the substrate. In general, diamond growth nucleates very poorly (i.e., slowly) on a substrate because of the high surface energy of diamond and the relatively low sticking probability of the precursor(s) for diamond nucleation.
C. References
Examples of prior art references dealing with diamond nucleation densities include the following.
U.S. Pat. No. 5,075,257, issued Dec. 24, 1991 to Hawk et al., discloses a method for the deposition of silicon and the formation of silicon films. Silicon powder of optimum particle size is aerosolized, charged, and then electrostatically deposited onto high melting point substrates, which may include semiconducting, insulating, and conducting materials such as silicon, sapphire, and molybdenum, respectively.
xe2x80x9cFocused ion-beam crater arrays for induced nucleation of diamond filmxe2x80x9d, A. R. Kirkpatrick et al., J. Vac. Sci. Technol. B 7 (6), November/December 1989, discloses the use of focused ion-beam (FIB) systems to ion-beam mill (sputter) shallow, small craters in precisely defined, reproducible patterns and spacings, and site densities of 1 per xcexcm2 or greater, with these craters causing nucleation.
xe2x80x9cEarly formation of chemical vapor deposition diamond filmsxe2x80x9d, S. Iijima et al., Appl. Phys. Lett. 57 (25) Dec. 17, 1990, discloses pretreatment of sonication of a silicon wafer in a water suspension of 10 micron-sized diamond powder, resulting in deposition of diamond powder in the range of 1010 to 1011 per cm2 on the wafer.
xe2x80x9cSurface And Interface Effects In The Nucleation And Growth Of Diamondxe2x80x9d, K.V. Ravi et al., New Diamond Science and Technology, 1991 MRS Int. Conf. Proc., discloses deposition of thin diamond like carbon (DLC) films on non-diamond surfaces is shown to substantially increase the nucleation density as well as facilitate the control of the microstructure of thick diamond films.
xe2x80x9cGeneration of diamond nuclei by electric field in plasma chemical vapor depositionxe2x80x9d, S. Yugo et al., Appl. Phys. Lett. 58 (10), Mar. 11, 1991, discloses a predeposition plasma chemical vapor deposition (xe2x80x9cCVDxe2x80x9d) process of several minutes duration that is implemented prior to the normal CVD process. For the predeposition step, the methane fraction in the methane-hydrogen feed gas is increased and at the same time an electric field is applied to the substrate. Nucleation densities as high as 1010/cm2 are obtained. After the predeposition process, diamond growth is carried out under normal CVD conditions.
xe2x80x9cNucleation of diamond on silicon, SiAlON, and graphite substrates coated with an a-C:H layerxe2x80x9d, J. J. Dubray et al., J. Vac. Sci. Technol. A, Vol. 9, No. 6, November/December 1991, discloses the deposition of an hydrogenated amorphous carbon layer on a substate surface to enhance nucleation of diamond.
xe2x80x9cCatalyst effect for diamond nucleation in the low pressure processxe2x80x9d, K. Kobayashi et al., Materials and Manufacturing Processes, 7 (3) 395-403 (1992), discloses the use of a substrate coated with catalytic materials by vacuum deposition, in a hot-filament chemical vapor deposition process, to achieve nucleation densities an order of magnitude higher than untreated substrates.
xe2x80x9cThermodynamics and kinetics for nucleation of diamonds in the chemical vapor deposition processxe2x80x9d, N. M. Hwang et al., Diamond and Related Materials, 1(1992) 191-194, discloses that the nucleation intensity ratio of the stable to the metastable phase of diamond is shown to be critically affected by the variation of the specific surface energy ratio of the two phases.
xe2x80x9cNucleation mechanisms of diamond in plasma chemical vapor depositionxe2x80x9d, S. Yugo et al., Diamond and Related Materials, 2 (1992) 328-332, discloses the application of a negative bias to silicon, leading to the formation of diamond precursors, and a nucelation density as high as 1010/cm2.
xe2x80x9cDiamond film nucleation and interface characterizationxe2x80x9d, P Bou et al., J. Mater. Res., Vol. 7, No. 8, August 1992, discloses an in situ pretreatment of a substrate in a plasma where large carbon carrier fluxes toward the surface are used, to minimize nucleation delay and enhance nucleation density.
xe2x80x9cNucleation and growth of diamond on carbon-implanted single crystal copper surfacesxe2x80x9d, T. P. Ong et al., J. Mater. Res., Vol 7, No. 9, September 1992, discloses prior to diamond nucleation, the modification of a single crystal copper surface by carbon implantation.
xe2x80x9cDiamond crystallite formation on Si(100) from the gas phase: Seeding or heterogeneous nucleation?xe2x80x9d, E. Molinari et al., Appl. Phys. Lett. 61 (11), Sep. 14, 1992, studied depostion of diamond onto silicon wafers scratched with diamond paste to study nucleation enhancement.
xe2x80x9cInvestigation of surface preparation for diamond deposition on molybdenum substrates by secondary ion mass spectrometryxe2x80x9d, R. Steiner et al., Diamond and Related Materials, 2 (1993) 958-962, discloses a nucleation pretreatment of polishing of the surface of molybdenum substrates with SiC and diamond powder.
xe2x80x9cEffect of substrate pretreatment on diamond depositionxe2x80x9d, Diamond and Related Materials, H. Maeda et al., 2 (1993) 758-761, discloses pretreatment of an Si wafer with diamond, c-BN, MoB, LaB6, TaB2 or Si powder suspended in acetone by an ultrasonic method.
xe2x80x9cElectrophoretic Deposition of Diamond Monolayers for CVD Growth of Diamond Thin Filmsxe2x80x9d, J. L. Valdes, B. A. Tao, J. W. Mitchell, G. W. Kammlott, and L. Seibles, May 16-21, 1993, a method for nucleating the growth of chemical vapor deposition diamond films on silicon by electrophoretic deposition from either aqueous or non-aqueous dispersions containing sub-micron colloidal diamond particles to achieve nucleation densities on the order of 1010/cm2.
xe2x80x9cHigh Density Diamond Nucleation On Unseeded Substrates By A Combined Microwave And Radio-Frequency Plasmaxe2x80x9d, May 16-21, 1993, discloses a microwave assisted chemical vapor deposition process in which nucleation was accomplished by applying an RF induced negative DC bias at a substrate that was immersed in a microwave plasma of a high methane concentration diluted by hydrogen.
xe2x80x9cNucleation And Growth Of Highly Transparent Nanocrystalline Diamond Filmsxe2x80x9d, W. Dotter, R. Erz, K. Jung and H. Ehrhardt, May 16-21, 1993, discloses nanocrystalline, optically transparent diamond membranes grown by microwave plasma deposition, with nucleation densities of 1010 nuclei/cm2 obtained by scratching silicon and quartz substrates with 10 nm diamond powder, and nucleation densities exceeding 1011 nuclei/cm2 with the dc-bias method.
xe2x80x9cEvidence for nonclassical nucleation at solid surfaces in diamond deposition from the gas phasexe2x80x9d, M. Tomellini, J. Mater. Res., Vol. 8, No. 7, July 1993, discloses a two-step kinetic model which gives the non-steady-state nucleation density function in terms of the rate constants for the active site to germ, germ to active site, and germ to kinetic steps.
xe2x80x9cNucleation of oriented diamond films on nickel substratesxe2x80x9d, P. C. Yang et al., J. Mater. Res., Vol. 8, No. 8, August 1993, discloses a seeding and multistep deposition process to nucleate and grow diamond films directly on nickel substrates in a hot filament chemical vapor deposition system.
xe2x80x9cQuantitative nucleation and growth studies of PACVD diamond film formation on (100) siliconxe2x80x9d, R. A. Bauer et al., J. Mater. Res., Vol. 8, No. 11, November 1993, discloses various surface treatments prior to microwave plasma assisted chemical vapor deposition of diamond films, including, diamond polishing, SiC scratching, alumina scratching, graphite powder wiping, and spin coating with polymethyl methacrylate.
xe2x80x9cDiamond nucleation on pretreated substratesxe2x80x9d, K. Kobayashi et al., Diamond and Related Materials, 2 (1993) 278-284, discloses use of an iron thin film to enhance nucleation density.
xe2x80x9cInitial stages in the growth of polycrystalline diamond on siliconxe2x80x9d, R. Stockel et al., Diamond and Related Materials, 2 (1993) 1467-1472, is a study of diamond films grown on crystalline silicon wafers scratched with a diamond suspension.
xe2x80x9cInfluence on diamond nucleation of the carbon concentration near the substrate surfacexe2x80x9d, D. Michau et al., Diamond and Related Materials, 2 (1993) 19-23, discloses the most useful method to promote nucleation is to scratch the substrate with diamond paste.
xe2x80x9cEpitaxial nucleation, growth and characterization of highly oriented, (100)-textured diamond films on siliconxe2x80x9d, B. A. Fox et al., Diamond and Related Materials, 3 (1994) 382, 387, discloses the use of bias enhanced nucleation in a multistep growth process for producing highly oriented, (100)-textured diamond films.
xe2x80x9cDiamond nucleation on surfaces using carbon clustersxe2x80x9d, R. J. Meilunas et al., J. Mater. Res., Vol. 9, No. 1, January 1994 discloses the use of fullerene films sublimated onto various non-diamond substrates to produce nucleation sites suitable for the formation of continuous diamond film.
xe2x80x9cEvaluation of a substrate pretreatment for hot filament CVD of diamondxe2x80x9d, J. Mater. Res., K. L. Menningen et al., Vol. 9, No. 4, April 1994, discloses measurement of the time evolution of both the methyl radical density and the acetylene mole fraction during hot filament chemical vapor deposition of diamond film.
xe2x80x9cSelective growth of diamond using an iron catalystxe2x80x9d, Y. Shimada et al., Diamond and Related Materials, 3 (1994) 403-407, discloses growth of diamond carried out on a silicon substrate with patterned iron films using chemical vapor deposition.
xe2x80x9cDiamond nucleation on nickel substrates seeded with non-diamond carbonxe2x80x9d, Yang et al., J. Mater. Res., Vol. 9, No. 5, May 1994, discloses the use of graphite and C60 powders to promote diamond nucleation. Prior to chemical vapor deposition, the nickel substrates were immersed into a suspension of either powders, and a layer of the carbon powder would be subsequently formed on the nickel surface when the substrates were removed from the suspension.
xe2x80x9cNanocrystal seeding: A low temperature route to polycrystalline Si filmsxe2x80x9d, J. R. Health et al., Appl. Phys. Lett., Vol. 64, No. 26, Jun. 27, 1994, discloses the use of eximer laser photolysis of disilane in a room temperature flow cell to produce silicon nanocrystals as seeds in making polycrystalline silicon films.
xe2x80x9cRole of the nucleation step in the growth rate of diamond filmsxe2x80x9d, L. Fayette et al., Diamond and Related Materials, 3 (1994) 480-485, discloses, for the well-known microwave-plasma-enhanced chemical vapor deposition process, the influence of substrate temperature and of the methane concentration in the gas phase either on the diamond nucleation step or on the whole growth process.
xe2x80x9cThe effect of ballpoint ink coating on the nucleation enhancement of low-pressure diamondxe2x80x9d, Peng XiLing et al., J. Mater. Res., Vol. 9, No. 6, June 1994, discloses the coating of silicon with ballpoint pen ink to increase diamond nucleation density.
xe2x80x9cDiamond nucleation by carbon fibers on unscratched substrate by hot-filament chemical vapor depositionxe2x80x9d, Nakamura et al., J. Mater. Res., Vol. 9, No. 7, July 1994, discloses the use of carbon fibers placed on unscratched substrates to nucleate diamond particles. The carbon fibers with approximate 7 xcexcm diameter were fixed onto the silicon substrate by being held down with stainless steel plates at each end of the fibers prior to deposition.
xe2x80x9cA pretreatment process for enhanced diamond nucleation on smooth silicon substrates coated with hard carbon filmsxe2x80x9d, Z. Feng et al., J. Mater. Res., Vol. 9, No. 8, August 1994, discloses pretreatment of unscratched silicon substrates with a methane-rich hydrogen plasma at a relatively low temperature for an hour to achieve diamond nucleation densities on the order of 108/cm2.
xe2x80x9cNucleation and growth during the chemical vapor deposition of diamond on SiO2 substratesxe2x80x9d, J. Rankin et al., J. Mater. Res., Vol. 9, No. 8, August 1994, discloses scratching of silicon and fused silica substrates with diamond paste to enhance nucleation density.
xe2x80x9cA modelling of diamond nucleationxe2x80x9d, S. Yugo et al., Diamond and Related Materials 4 (1995) 903-907 (presented Sep. 25-30, 1994, Diamond Films ""94, Il Ciocco, Italy), discloses the use of negative bias on silicon substrate during microwave chemical vapor deposition of diamond.
xe2x80x9cNucleation and initial growth of diamond film on Si substratexe2x80x9d, N. Jiang et al., J. Mater. Res. Vol 9, No. 10, October 1994, discloses scratching of polished silicon substrates with diamond paste to enhance nucleation density.
xe2x80x9cUltrahigh nucleation density for growth of smooth diamond filmsxe2x80x9d, G. S. Yang et al., Appl. Phys. Lett. 66 (3), Jan. 16, 1995, and xe2x80x9cEffect of ultrahigh nucleation density on diamond growth at different growth rates and temperaturesxe2x80x9d, G. S. Yang et al., J. Vac. Sci. Technol. B. Vol. 13, No. 3, May/June 1995, both disclose an extremely high nucleation density, calculated to be as high as 1.599xc3x971011/cm2, was achieved by coating 0.038 xcexcm diamond powder on the surface of the silicon substrate.
xe2x80x9cEffect of the substrate state on the formation of diamond film in a low temperature microwave-plasma-enhanced chemical vapor deposition systemxe2x80x9d, S. H. Kim et al., J. Vac. Sci. Technol. A, Vol. 13, No. 3, May/June 1995, discloses that low temperature deposition can be achieved only by making the substrate position remote from the plasma under the normal chemical vapor deposition conditions. Nucleation was enhanced by pretreatment of the substrate surface with 30 micron diamond powder-acetone solution in an ultrasonic cleaner.
xe2x80x9cEnhancement of diamond nucleation by ultrasonic substrate abrasion with a mixture of metal and diamond particlesxe2x80x9d, Y. Chakk et al., Appl. Phys. Lett. 66(21), May 22, 1995, discloses that the nucleation density obtained by ultrasonic abrasion with diamond powders alone can be enhanced by using a mixed slurry of diamond and metal powders. The metal powders utilized were W, Ta, Mo, Nb, Ti, Al, Fe, Ni, Cu and Si. Nucleation densities were on the order of 107 to 109 per cm2.
xe2x80x9cMechanisms for the ion-assisted nucleation of diamondxe2x80x9d, S. McGinnis, May 1995 ECS Meeting 4th International Symposium on Diamond Materials, discloses the effects of substrate bias voltage, electrically isolated substrate regions, and substrate temperature on nucleation density rate, for an ion-assisted diamond nucleation process.
xe2x80x9cSi+ implantation: A pretreatment method for diamond nucleation on a Si waferxe2x80x9d, J. Yang et al., Appl. Phys. Lett. 66(24), Jun. 12, 1995, discloses pretreatment of silicon wafer with Si+ ion beam implantation for diamond nucleation.
xe2x80x9cEvidence of an energetic ion bombardment mechanism for bias-enhanced nucleation of diamondxe2x80x9d, S. McGinnis et al., Appl. Phys. Lett., 66(23) June 1995, discloses a bias-enhanced nucleation of diamond to produce nucleation densities of 1010/cm2.
xe2x80x9cDiamond Nucleation And Growth On mirror-Polish Silicon Wafter Pretreated By Silicon Ion Implantationxe2x80x9d, Jie Yang et al., Applications of Diamond Films and Related Materials: Third International Conference, 1995, discloses diamond films obtained by the hot-filament chemical vapor deposition method on silicon wafer, with the substrates pre-implanted by a silicon ion beam.
xe2x80x9cSeeding With Purified Ultrafine Diamond Particles For Diamond Synthesis By CVDxe2x80x9d, H. Makita et al., Applications of Diamond Films and Related Materials: Third International Conference, 1995, discloses seeding of silicon substrates with purified nanocrystal diamond particles about 5 nm in diameter by first removing the substrate surface oxidized layer with 10%. HF acid, and then dipping the substrates into a alcohol/water colloidal solution of the particles.
However, in spite of these advancements in the prior art, there is still room for improvement in providing diamond films on non-diamond substrates.
Thus, there is still a need for a substrate treating process that provides higher nucleation densities.
There is another need in the art for a substrate treating process that provides for a more uniform and reproducible distribution of the nucleation sites on the substrate.
There is even another need in the art for a treating process that can seed complex geometric shapes.
These and other needs in the art will become apparent to those of skill in the art upon review of this specification, including its drawings and claims.
It is an object of the present invention to provide for a substrate treating process that provides higher nucleation densities.
It is another object of the present invention to provide for a substate treating process that provides for a more uniform and reproducible distribution of the nucleation sites on the substrate.
It is even another object of the present invention to provide for a treating process that can seed complex geometric shapes.
These and other objects of the present invention will become apparent to those of skill in the art upon review of this specification, including its drawings and claims.
According to one embodiment of the present invention there is provided a method of seeding substrates for subsequent coating. The method utilizes electrostatic spraying to provide charged particles on a substrate. The method generally includes aerosolizing and electrostatically charging the seed particles with an electrostatic charge of one polarity. The substrate is then provided with a polarity suitable to attract the particles. The charged particles are then contacted with the substrate to form a seeded substrate having particles electrostatically affixed thereto. According to a further embodiment of this embodiment, the seeded substrate is then subjected to heat treatment to form the affixed particles into a layer. According to another further embodiment of this embodiment, deposition techniques are utilized to form a layer on the seeded particles.
According to another embodiment of the present invention there is provided a method of seeding substrates for subsequent coating. The method utilizes electrostatic spraying to provide charged particles on a substrate. The method generally includes dispersing the seed particles in a liquid to form a dispersed solution. The dispersed solution is then aerosolized and the seed particles electrostatically charged with an electrostatic charge of one polarity. The substrate is then provided with a polarity suitable to attract the particles. The charged particles are then contacted with the substrate to form a seeded substrate having particles electrostatically affixed thereto. According to a further embodiment of this embodiment, the seeded substrate is then subjected to heat treatment to form the affixed particles into a layer. According to another further embodiment of this embodiment, deposition techniques are utilized to form a layer on the seeded particles.
According to even another embodiment of this present invention, there is provided a seeded substrate having a particle density greater than about 5xc3x971011 particles per cm2, which substrate is useful as a deposition precurser.
According to still another embodiment of this present invention, there is provided a seeded substrate having particles electrostatically affixed to the substrate, which substrate is useful as a deposition precurser.
According to yet another embodiment of the present invention, there is provided a substrate supporting a polycrystalline layer, with the polycrystalline layer having a nucleation density greater than about 5xc3x971011 per cm2.
According to even still another embodiment of the present invention, there is provided an apparatus for seeding. The apparatus includes a reservoir of liquid and seeding particles, and a disperser for dispersing the seeding particles in the liquid. The apparatus further includes an electrostatic spray gun having a corona tip. A gas blower propels and provides the seeding particles to the corona tip. The apparatus even further includes a substrate holder for holding and positioning a target substrate at the discharge of the corona tip.
According to even yet another embodiment there is provided an apparatus for depositing a layer onto a substrate. The apparatus includes an electrostatic seeding sprayer to deposit a seed layer of charged particles from a reservoir of particles and liquid onto the substrate, and a deposition section for depositing a layer onto the seed layer. In a more specific embodiment, the apparatus further includes a disperser for dispersing the particles in the liquid.